1. Field of the Invention
The present invention relates to an imaging lens. In particular, the present invention relates to an imaging lens having a two-lens structure suitable for size and weight reduction, and improved optical performance. The imaging lens is used in a camera that forms an image of an object, such as scenery and human figures, on an image-taking surface of an image sensor element, such as a charge-coupled device (CCD) and a complementary metal oxide semiconductor (CMOS), mounted on a portable computer, a television phone, a portable phone, and the like.
2. Description of the Related Art
In recent years, there has been an increasing demand for cameras utilizing a solid image sensor element, such as the CCD and the CMOS, that are mounted on a portable phone, a portable computer, a television phone, and the like. It is demanded that a camera such as this be small and light because the camera is required to be mounted on a limited installation space.
Therefore, an imaging lens used in a camera such as this is also required to be similarly small and light. Conventionally, a single-lens structure lens system using a single lens has been used as such an imaging lens.
However, although a single-lens structure lens system such as this is optimal for size and weight reduction, the lens system cannot sufficiently utilize the resolution capabilities of recent solid image sensor elements (such as SXGA) having a high resolution of one million pixels or more.
At the same time, in a three-lens structure lens system, an extremely high optical performance can be achieved because each aberration leading to deterioration of optical performance can be effectively corrected. However, a three-lens structure lens system such as this has a large number of components, making size and weight reduction difficult. Production cost also increases because each component requires high precision.
On the other hand, although optical performance that is as high as that of the three-lens structure lens system cannot be expected of the two-lens structure lens system, a higher optical performance than that of the single-lens structure lens system can be achieved. The two-lens structure lens system is compact and suitable for a high-resolution solid image sensor element.
As a two-lens structure lens system such as this, for example, the lens systems described in the following Patent Literatures 1, 2, and 3 have been used.    [Patent Literature 1]
Japanese Patent Unexamined Publication No. Heisei 4-211214    [Patent Literature 2]
Japanese Patent Unexamined Publication No. 2003-107344    [Patent Literature 3]
Japanese Patent Unexamined Publication No. 2004-62014
However, in the lens system described in Patent Literature 1, a second lens is a meniscus lenses having little power whose convex surface faces an object side. Therefore, correction of magnification chromatic aberration is difficult.
In the lens system described in Patent Literature 2, a first lens has negative power. Therefore, size and weight reduction is difficult. Moreover, a diaphragm is disposed on an image surface side of a second lens. Therefore, telecentricity is poor.
In the lens system described in Patent Literature 3, although a diaphragm is disposed closest to the object side, a second lens has a strong concave surface on an image surface side. Therefore, telecentricity is poor.